Color image scanning is a process of converting an image, recorded on either a transparency or a reflective print, to an electronic image. Scanning, as such, is commonly employed as a first step in the transmission of an image from one storage medium to another, or in the enhancement or the analysis of the image prior to its transmission.
With an image-bearing sheet held in a given plane, a color image scanner measures the optical density of the image by illuminating the sheet with polychromatic light. Commonly, a color image scanner measures the amount of light in a given color space, e.g. red (R), green (G), and blue (B), transmitted through, or reflected from, the image-bearing sheet. In doing so, the scanner effectively divides the image into discrete picture elements, or pixels, and assigns to each a number or value representing an average density for each color measured. Commonly, the pixels are arranged in rows and columns to form a two-dimensional grid with the density of each pixel corresponding to a relatively small portion of the overall image.
As is appreciated by those skilled in the image scanner art, calibrating the spectral sensitivity of a color image scanner is important to provide accurate reproduction of all colors in the image.
The spectral sensitivity of a color image scanner is determined by the spectral characteristics of a variety of scanner components such as the spectral content and sensitivity of its light source, the spectral transmissivity characteristics of filters, lenses and the like, and the spectral sensitivity of its image sensor. All of these factors, however, are likely to change, component to component, particularly with the use of a scanner over long periods of time. Furthermore, even with complete component stability, an original component may be replaced from time to time with a new component having a somewhat different characteristic.
A known technique for performing a calibration operation on a color film scanner is described in U.S. Pat. No. 4,898,467, which was issued in the name of James R. Milch to the assignee of the present invention, and which is expressly incorporated herein by reference. A spectrometer is disclosed for self-calibrating a color image scanner of the line scanner or area scanner type. It includes a member, having an optical slit, movable into position on an optical axis of the scanner between its polychromatic light source and its focusable lens in a plane occupied by a color image when it is scanned. A diffraction grating is similarly movable onto the optical axis between the focusable lens and an image sensor. The light source illuminates the slit and the diffraction grating disperses transmitted polychromatic light from the slit according to its wavelength, forming duplicate spectra off-axis across respective halves of the image sensor, with longer wavelengths being directed to respectively higher angles.
Each pixel of the image sensor receives an amount of light energy corresponding to its off-axis position, the repetition frequency of rulings of the grating, and the spectral sensitivity of the scanner itself. Accordingly, knowing the off-axis position of an image sensor pixel and knowing the grating frequency, the profile of light energy across the image sensor is a direct measure of the spectral response of the scanner. The calibrating of the color image scanner, based on its spectral response, can take place in signal processing electronics at the output of the image scanner.
While the Milch patent describes a calibrating system that has proved successful in certain applications, it nonetheless presents a problem in other applications where it is mechanically difficult to have an element, such as a diffraction grating, that is movable in and out of a position very near an image sensor.